No-brainer here. Buy the used one. The new one says "Ultrasonic", but what is, is Micro USM, which isn't that great. The old one (ALL 28-105 f3.5-4.5 versions) use the metal mount. The key here is the maximum f-stop range- the good ones are f3.5-4.5- every one of those are the better built versions and they are still available new. The 28-105's that are f4-5.6 lenses are cheap, all plastic affairs and come automatically in the various "Kit" versions of the Elan 7 because they are so cheap. The "Real" USM motors, the very best ring USM motors, are mounted on the EF lenses with the gold bands around the lens barrel. AF performance in lenses that utilize the very best ring USM is spectacular. They AF in virtual silence and just literally jump into focus. The f4-5.6 versions crawl into focus when you compare the two and the build quality is quite inferior.

Thanks, Im bidding on one on Ebay through a guy Ive got nice condition lenses through before. Im not keen on any of the plastic new lenses Ive looked at. The main selling point I see for them is the light weight, but they look cheap and nasty to me. Thanks for the info regarding the different kinds of USM, I never knew that there was a difference.

That's Canon's marketing strategy- to sell those micro USM lenses as "USM". Many users do not know the difference and Canon uses that to sell a lot of junk. They almost never use the term "Micro USM", and where they do it's just barely skimmed over.

Canon is not alone in doing this kind of thing. Today Nikon cashes in on the "ED" moniker. Used to if you bought an 'ED" Nikkor it was not only superior optically, it was also a heavy duty, pro lens. Today they sell as much plastic junk with that term inscribed on it as they do nice optics.